Parvalbumin (PV)-immunoreactive neurons in rat neostriatum were studied under light and electron microscopes. A small number of neurons in the striatum were immunoreactive for PV (a Ca-binding protein). Most of them were also strongly immunoreactive for glutamate decarboxylase but were negative for NADPH-diaphorase activity. Light microscopic analysis revealed that PV-containing neurons have somata with fusiform or polygonal shape and are medium to large in size. The dendrites were smooth and cylindrical at the proximal portion but were varicose at the distal portion. Thin PV-immunoreactive fibers with large boutons were unevenly distributed in the striatum. Electron microscopy revealed that the somata of PV-immunoreactive neurons had a deeply indented nucleus with a nucleolus and often an intranuclear rod. These are the morphological features reported for interneurons of the striatum. Gap junctions formed between two neighboring PV-immunoreactive dendrites. A total of 175 boutons forming synapses with somata and dendrites of PV-immunoreactive neurons were examined. Of these, 115 were small in diameter (less than 1 micron), contained densely packed round vesicles and formed asymmetrical synapses mainly with dendrites. The other 60 boutons formed symmetrical synapses with somata and dendrites of PV-immunoreactive neurons. Both myelinated and unmyelinated axons with boutons were observed. PV-immunoreactive boutons had a diameter of 0.3-2 microns and contained round or elongated vesicles which were about 35 nm in diameter. The boutons formed symmetrical synapses with postsynaptic targets. Of the 100 PV-immunoreactive boutons, 51 were found on somata and proximal dendrites of medium-sized neurons containing a large, round, centrally located nucleus. The others formed synapses with dendrites of various sizes. It was occasionally observed that varicose dendrites free of spines were contacted by a large number of PV-immunoreactive boutons. The study indicates that, in the striatum, immunocytochemistry for PV selectively stains GABAergic interneurons and that the GABAergic interneurons are incorporated in a feed-forward inhibitory circuit of the striatum.